Numerous advanced coal cleaning processes have been developed in recent years. The main objectives of the various processes are two-fold: 1) to remove the impurities in the coal to the extent that it contains a very small amount of ash; and 2) to remove the sulfur to minimize the SO.sub.x emissions during combustion.
The resulting superclean (&lt;2% ash) or ultraclean (0.8% ash) coals can be used to displace the oil and gas used in utilities and possibly for other applications. For coals containing mainly inorganic sulfur, a single advanced coal cleaning process may be able to meet both of the objectives. However, for those containing large amounts of organic sulfur, chemical or microbial coal cleaning processes may be necessary.
If the sulfur removal is still not in compliance with emission standards, scrubbers may have to be used in conjunction with the advanced coal cleaning processes. Since scrubbing is costly both in terms of capital and operation and maintenance (O&M) costs, there is an advantage to removing as much sulfur as possible prior to combustion. As a rule of thumb, the cost of conventional wet-scrubbing is estimated to be in the range of $750.00 to $1000.00/ton of SO.sub.2 removed. If a utility burns a coal containing 2% sulfur, for example, it will have to spend $27.00 to $36.00/ton of coal, assuming that 90% of the sulfur is removed by scrubbing. If, on the other hand, the same coal is cleaned by an advanced coal cleaning process to 1.4% sulfur and the remaining sulfur is removed by a less costly, although less efficient, scrubbing technique, the utility can reduce the cost of scrubbing substantially. The duct injection process is regarded as one such technique that can remove approximately 70% of the sulfur at a cost of approximately $500.00/ton of SO.sub.2 removed. In this case, the utility can spend only $9.80/ton of coal burned. A simplistic calculation as such may justify a combined use of advanced coal cleaning and scrubbing techniques. It also suggests that the cost of pre-combustion coal cleaning should not exceed $17.00-26.00/ton of cleaned coal to be able to compete against the conventional wet-scrubbing techniques.
Of the various advanced coal cleaning processes being developed, the oil agglomeration process may be one of the most promising techniques. It is based on the fact that higher rank coals are more wettable in oily substances than the associated mineral matter. Thus, if an oil is added to an aqueous suspension of pulverized coal, the coal particles will be collected into the oil phase, while the mineral matter will remain in the aqueous phase, allowing the two to be separated from each other. When a sufficient amount of oil is added, the coal particles form agglomerates larger than 1 to 2 mm in diameter, which can be effectively separated from the dispersed mineral matter by screening.
The oil agglomeration process described above is simple and efficient, and the product coal shows improved dewatering characteristics. However, the process suffers from one problem, that is, high oil consumption. Typically, 10% or more of oil by weight of feed solids is required for cleaning coal containing large portions of -325-mesh material. At this rate, the cost of oil alone can easily make the process uneconomical as compared to the wet-scrubbing techniques. Therefore, a continuing battle has been waged in recent years to reduce the oil consumption. Stripping with superheated steam can recover the spent oil from the clean coal, but the reduction in O&M cost is estimated to be only 28% (Cheh et al, "Solvent Recovery for the Oil Agglomeration Coal Cleaning Process," SME-AIME Annual Meeting, Dallas, Tex., February 14-18, Pre-print No. 82-48 (1982)). This reduction is not enough to make the process competitive. More recently, Capes, "Liquid Phase Agglomeration: Process Opportunities for Economic and Environmental Challenges," Challenges in Mineral Processing, K.V.S. Sastry et al, Society of Mining Engineers, Littleton, Colo., pp. 237-251 ( 1989), reported that as little as 1% oil is sufficient if the agglomerates are separated by flotation. Nevertheless, the 1% oil is still an order of magnitude higher than what is normally required in conventional or advanced flotation processes.
Perhaps the most intriguing method of reducing the oil consumption is to use an agglomerant that can be readily recovered and recycled to the process. Otisca Industries, Inc., is now using pentane which has a boiling point of 36.degree. C. and a low surface tension (17.3 dyne.cm). The amount of pentane used during the process of agglomeration is about 50% by volume of the recovered coal, but most of it is recovered for recycle by heating the product at 50.degree. C. Using this process, Keller et al, "The Demineralization of Coal Using Selective Agglomeration by the T-Process," Coal Preparation, 6:(in press), (1989), showed that of the more than one hundred different coal samples tested, more than half yielded a product coal with ash contents less than 1%, and several were cleaned to less than 0.3% ash. The Btu recovery and pyritic sulfur rejection were both greater than 95%. Another example of using reusable agglomerant is the LICADO process, in which liquid carbon dioxide is contacted with an aqueous coal slurry at about 850 psi. There are other processes that are also designed to cut down the reagent cost by using reusable agglomerants. However, a common drawback of such effort is that the capital and the O&M costs are high. Furthermore, the potential hazards of using high-vapor pressure hydrocarbons and asphyxial gases such as carbon dioxide may hinder widespread use in commercial-scale operations.
The major advantage of the oil agglomeration process is that it is capable of recovering coal particles as small as a few microns in diameter or less. The ability to separate micron-sized particles is an important advantage when a coal must be pulverized to very fine sizes in order to liberate the mineral matter and pyritic sulfur more completely. It would, therefore, be advantageous to further improve the conventional oil agglomeration process to the extent that the oil consumption is no longer the impediment to upgrading coal, 0 graphite or any other naturally hydrophobic substances commercially.
For this reason, a novel agglomeration process has been developed, in which in accordance with the present invention no oily agglomerant is required for separating fine particles of hydrophobic material from the associated hydrophilic ones. The process is autogenous in that it requires virtually no reagent. It is based on an improved understanding of the most fundamental mechanisms by which the stability of aqueous suspensions are controlled. For the reasons described hereinafter, the process in accordance with the present invention has been named "selective hydrophobic coagulation."